1. Field of the Invention
The present invention relates to a boron-nitride-containing material and a method thereof.
2. Description of the Related Arts
Hexagonal boron nitride is known to be a chemically stable material which has a high heat conductivity, an excellent electrical insulating property, and an excellent lubricating property and does not react with a molten product of iron, copper, nickel, zinc, gallium, arsenic, glass, cryolite, etc. Also, hexagonal boron nitride is stable in air until 950.degree. C. and in an inert gas or nitrogen gas atmosphere until 2,200.degree. C. Also, hexagonal boron nitride has the feature that machining such as cutting, grinding, etc., can be easily performed as in the case of metals. Boron nitride itself or material containing boron nitride has been supplied for various uses by utilizing that feature.
The uses as the powder of hexagonal boron nitride, has been used as additives for plastics, lubricants, mold releasing agents for aluminum die casting and a glass molding, etc. Also, sintered hexagonal boron nitride, has been used for insulating parts, heat-resistant parts, crucibles for molten metals, brake rings for horizontal continuous casting, heat-emitting members, setters for sintering the powder moldings of metals or ceramics, mold materials, etc. Also the application thereof for refractories for casting, such as upper nozzles, immersion nozzles, etc., has been attempted.
As a method of synthesizing the powder of hexagonal boron nitride having a wide use as described above, the following methods are known.
(1) A method of synthesizing the powder thereof by heating borax and urea to a temperature of at least 800.degree. C. in an ammonia atmosphere as described in JP-B-38-1610 (the term "JP-B" as used herein means "examined Japanese Patent Publication"). PA1 (2) A method of synthesizing the powder thereof by heating a mixture of boric acid or boron oxide and calcium phosphate in an ammonia atmosphere as descried in JP-B-42-24669. PA1 (3) A method of synthesizing the powder thereof by heating boric acid and a nitrogen-containing compound (urea, melamine, dicyandiamide, etc.) to a temperature of at least 1600.degree. C. as described in JP-B-48-14559. PA1 preparing a mixed powder including a boride and a first oxide, the boride consisting of boron and another element; PA1 heating the mixed powder in a nitriding atmosphere to reduce the first oxide by said another element in the boride, to a boron nitride and at least one selected from the group consisting of a second oxide having less bonded oxygen than the first oxide, a oxynitride, a nitride and a boride, resulting in a boron nitride containing material. PA1 preparing a mixed powder including at least two borides; and PA1 heating the mixed powder in a nitriding atmosphere to produce a boron nitride and a compound which comprises elements bonding to boron in said at least two borides. PA1 preparing a mixed powder including a boride and Al; and PA1 heating the mixed powder in a nitriding atmosphere to produce a boron nitride and AlN.
Also, a boron nitride powder synthesized at a low temperature of 1400.degree. C. or less contains unreacted materials, boron oxide, and B--N--O series intermediates in addition to boron nitride (BN) and also the boron nitride powder has an insufficient crystallization and is chemically unstable. Thus, the crude boron nitride powder is once cooled and washed with water followed by drying, and subjected to a heat treatment again for purification and crystallization. As one of these methods, a method of mixing the powder with a carbonaceous powder and heating the mixture to a temperature of at least 1500.degree. C. in an ammonia gas is known as described in JP-A-61-256905 (the term "JP-A" as used herein means "unexamined Japanese patent application").
The boron nitride thus obtained has a difficult sintering property and does not shape a sintered product by itself. Thus, for producing a sintered product of boron nitride, it is general to add a sintering aid to such a boron nitride powder and apply thereto a hot press and, for example, a method of adding a borate of an alkaline earth metal to boron nitride and hot-pressing the mixture is known as described in JP-B-49-40124.
For the production of a sintered material of a boron-nitride-containing complex material, a hot press method is applied. For example, a method of obtaining a BN--TiB.sub.2 --AlN three-component series container for metal vapor deposition by hot-pressing a mixed powder composed of titanium diborate, boron nitride, boron carbide and metallic aluminum in vacuum is disclosed in JP-A-62-139866.
However, the hot-pressing method is problematic in that the production efficiency is poor and the method can be applied to a simple-shape part only and the industrial utilization of the method is limited. For example, it is very difficult to produce a long product such as a thermocouple protective tube of a boron-nitride-containing complex material.
Thus, a technique capable of normal-pressure sintering of these parts by using a complex material of boron nitride and other material(s) as such parts and also, at the same time, improving the sintering property thereof has been developed. For example, JP-A-61-132564 discloses a method of CIP molding a mixed powder of BN, Al.sub.2 O.sub.3, and B.sub.2 O.sub.3 and sintering the mixed powder in a non-oxidizing atmosphere at normal pressure. In this way normal-pressure sintering is applied to the boron-nitride-containing complex material capable of being normal-pressure sintered as described above, the fault (the low productivity and the restriction on the shape of the parts) in the hot press method can be avoided.
Also, a method wherein a phenol resin is added, as a binder, to a boron-nitride-containing material, the material is molded, and thereafter the material is bonded with residual carbon obtained by thermal decomposition is known. For example, JP-A-3-268849 discloses a method of adding from 8 to 15% by weight a liquid phenol resin to a mixed powder of boron nitride and partially stabilized zirconia followed by kneading and decomposing the phenol resin by sintering the kneaded product at a temperature of from 1,000.degree. C. to 1,200.degree. C. to obtain an immersion nozzle for continuous casting.
Now, as described above, for the hexagonal boron nitride material, the market expansion of the powder or the sintered product has been expected owing to its specific characteristics, and the practical use thereof has proceeded owing to the technology as described above but the market thereof has not progressed at present as expected. The main cause thereof is that the hexagonal boron nitride powder is expensive.
The reason that the hexagonal boron nitride powder is expensive is that since the purity of boron nitride as the product is low because borax and boron oxide as the raw materials are softened in the course of the reaction during which they become glassy, which hinders the nitriding, and also the nitriding and the crystallization cannot be simultaneously carried out. Therefor, for the production of the hexagonal boron nitride powder, many steps are required as described above and the production steps become troublesome.
Thus, a method of forming hexagonal boron nitride in the sintered product without using the expensive hexagonal boron nitride powder has been proposed. For example, JP-A-4-325461 disclosed that by molding a mixed powder of silicon and B.sub.4 C and heating the mixed powder in a nitrogen gas atmosphere, a sintered product containing BN and C formed by the nitriding of B.sub.4 C, SiC formed by the carbonization of silicon, and Si.sub.3 N.sub.4 formed by the nitriding of silicon can be produced.
Also, JP-A-5-201771 discloses a method of forming a boron-nitride-containing sintered product by reaction sintering. Practically, using at least one material selected from the group consisting of silicon, SiB.sub.4, SiB.sub.6, AlB.sub.2, AlB.sub.12, CaB.sub.6, silicon oxide, and alumina as the starting powders, a molding having a shape resembling the final shape is obtained by casting and the molding is nitrided for from 5 to 100 hours, at a temperature of from 1,000.degree. C. to 1,400.degree. C., and a nitrogen gas pressure of from 500 to 1,000 mbar or is nitrided for from 1 to 20 hours at a nitrogen gas pressure of from 2 to 20 bar. By forming Si.sub.3 N.sub.4 from silicon and forming a nitride of an element bonded to boron and BN from the boride (SiB.sub.6 shapes Si.sub.3 N.sub.4 and BN, and AlB.sub.12 shapes AlN and BN) in the nitriding step, Si.sub.3 N.sub.4, BN, and AlN are formed. After the nitriding step, by sintering the products formed in the temperature range of from 1,500.degree. C. to 1,900.degree. C., .beta.'-sialon is formed from Si.sub.3 N.sub.4, AlN, and previously added Al.sub.2 O.sub.3 is formed and silicon oxynitride (Si.sub.2 N.sub.2 O) is formed from Si.sub.3 N.sub.4 and previously added SiO.sub.2, whereby a sintered product composed of .beta.'-sialon, acid silicon nitride, and boron nitride as the main components is obtained. In this case, it is described that, the carbon content of the boride is preferably required to be 0.1% or less and also B.sub.4 C is excluded from the objects. Also, it is described that oxide(s) such as alumina, silicon dioxide, mullite, zirconia, yttrium oxide, YAG (yttrium-aluminum garnet 3Y.sub.2 O.sub.3.5Al.sub.2 O.sub.3), calcium oxide, and magnesium oxide may be added to the starting materials, but B.sub.2 O.sub.3 must not be present. Also, it is limited that these oxide additives are not reacted with the remaining starting powders in the nitriding step and during the sintering treatment.
Both JP-A-4-325461 and JP-A-5-201771 disclose that a sintered product of the complex material containing silicon nitride and boron nitride is obtained by the nitriding of silicon and the boride(s) as described above.
Also, it is known that the powders are formed prior to the nitriding step as described in the patent publications mentioned above and, in particular, it is described in the latter that a shaped material having a shape resembling the final shape is formed by casting the mixture of the starting materials and then applying nitriding to the shaped product.
As described above, the market expansion of the hexagonal boron nitride material is expected as the powder or the sintered product thereof owing to the specific characteristics of the material but the market thereof has not expanded as expected by reason that the hexagonal boron nitride powder is expensive.
In regard to the problem, as to the sintered product, a method of using a boride powder in place of the hexagonal boron nitride powder as a raw material and forming a hexagonal boron nitride by the nitriding of a molding containing the boride powder to produce a complex material containing boron nitride is proposed in JP-A-4-325461 and JP-A-5-201771 as described above, and if an inexpensive boride is obtained, the method leads the solving of the cost problem described above.
However, the method is applied to a limited composition system only to obtain the sintered material of the complex material containing silicon nitride and boron nitride by the nitriding of metallic silicon and a boride and does not project the various possibilities of a boron-nitride-containing complex material.
Also, when various reactions given in JP-A-4-325461 and JP-A-5-201771 are carried out, a problem occurs in that the product exhibits a volume increase of from 20 to 80%. That is, as described in the patent publications, when the starting materials are molded and the foregoing reaction is performed on to the molding in the state of the free surface thereof, the molding is expanded and deformed and in this step, cracks occur in many cases. Accordingly, it is difficult to obtain a sintered product capable of being used practically as it is.